Semiconductor device

ABSTRACT

In a semiconductor device constituting a GaAs MESFET, a GaAs substrate is prepared from a base material containing boron and carbon ions as impurities having a total impurity concentration of 2×10 17  atoms/cm 3  or more. Electrode layers are formed at predetermined portions on the GaAs substrate, and an active layer is formed to be adjacent to the electrode layers by ion implantation. Source and drain electrodes are formed on the electrode layers, respectively, and a gate electrode is formed on the active layer.

This application is a continuation of now abandoned application Ser. No.07/556,880 filed Jul. 23, 1990, which in turn is a continuation of nowabandoned application Ser. No. 07/322,333, filed Mar. 10, 1989.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improvement of a semiconductordevice constituting a GaAs MESFET.

2. Description of the Related Art

Conventionally, Cr-doped semi-insulating GaAs or non-doped LEC (LiquidEncapsulated Czochralski) GaAs has been used as base material of a GaAsMetal-Semiconductor Field-Effect-Transistor (to be referred to as a GaAsMESFET hereinafter). In order to form a GaAs MESFET from this basematerial, the following manufacturing steps are conventionally required.

When a non-doped LEC GaAs substrate is used as a base material, a properpretreatment is performed for the substrate. Thereafter, an SiOx layerhaving a thickness of, e.g., 5,000 Å is deposited on the substrate bychemical vapor deposition. Openings are formed on desired portions ofthe substrate through the SiOx layer using a photolithography technique.Si ions are implanted through the openings at an accelerated energy of180 keV and a dose of 5 ×10¹³ ions/cm², to form electrode layerscorresponding to the openings in the substrate. After the resist patternincluding the SiOx layer is removed, a proper pretreatment is performedagain on the surface of the substrate, and another SiOx layer isdeposited by chemical vapor deposition. An opening is formed on thedesired portion of the substrate through the SiOx layer by using aphotolithography technique. Si ions are implanted through the opening atan accelerated energy of 100 keV and a dose of 3 ×10⁻¹² ions/cm², toform an active layer just under the opening in the substrate. Then, theresist pattern including the SiOx layer is removed. Thereafter, acapless annealing process is performed in an arsenic atmosphere at 850°for 15 minutes and the implanted Si ions are electrically activated.Finally, gate, source, and drain electrodes are formed on the substrateby a lift-off method.

As described above, it is known that a GaAs base material used in a GaAsMESFET obtained by conventionally-known manufacturing steps normallycontains boron and carbon atoms, and the activation ratio of theimplanted Si ions during a capless annealing process after an activelayer is formed depends upon the concentrations of boron and carbonions. For this reason, it has been attempted to reduce theconcentrations of residual impurities such as boron and carbon ions.However, it is known that when the total concentration of boron andcarbon ions contained in the base material is about 2 ×10¹⁷ atoms/cm³ orless, the drain current-drain voltage (ID-VD) characteristic curve ofthe manufactured GaAs MESFET represents a nonlinear operation, as shownat a point A in FIG. 1, at a predetermined voltage value, e.g., 3 V ormore and a drain current value of 100 μAM or more. It is considered thatthese nonlinear characteristics are caused because the interface betweenthe active layer formed by implantation of Si ions and the base materialin contact with the active layer is not clear but is blurred, and adepth profile is formed loosely extending downward from the portion nearthe lower end of the active layer. More specifically, conventional GaAsMESFET, if the drain voltage VD is increased, a leakage current flowingthrough a portion other than the active layer and the electrode layerformed in the substrate, in particular, a current flowing through theabove-mentioned blurred portion starts oscillating at or above apredetermined threshold value. It is considered that because thiscurrent serves as a gate, the above-mentioned nonlinear characteristicsare generated. The nonlinear operation of the drain current adverselyaffects the noise characteristics of the GaAs MESFET. In addition, it isconfirmed that the above problem is similarly posed on a GaAs MESFETmanufactured by forming an epitaxial layer on a base material.

As a means for solving the above problem, it has been already disclosedthat high-concentration Cr ions serving as an impurity doped in asubstrate can prevent the above-mentioned nonlinear operation, in "IEEETransaction on Electron Devices., Vol. ED-34, No. 6, Jun. 1987, pp.1239-1244". However, in this disclosed technique, the problem of a largechange in drain current ID over time is also pointed out.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the abovesituation, and has as its object to provide a semiconductor deviceconstituting a GaAs MESFET having advantages such as, e.g., a nonlinearoperation can be suppressed and a change in drain current does not occurover a period of time.

In order to achieve the above object, a semiconductor deviceconstituting a GaAs MESFET according to the present invention,comprises: a GaAs substrate prepared from a base material containingboron and carbon ions as impurities having a total impurityconcentration of 2 ×10¹⁷ atoms/cm³ or more; electrode layers formed onpredetermined portions of the GaAs substrate; an active layer formedadjacent to the electrode layers on the GaAs substrate by ionimplantation; and source and drain electrodes formed on the electrodelayers, respectively, and a gate electrode formed on the active layer.With the above arrangement, i.e., with a semiconductor deviceconstituting a GaAs MESFET using, as a base material, a GaAs substratecontaining boron and/or carbon ions at a total concentration of 2 ×10¹⁷atoms/cm³ or more, there is provided a semiconductor device havingadvantages such as, e.g., a nonlinear operation can be suppressed and achange in drain current does not occur over a period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing drain current-drain voltage characteristics ofa GaAs MESFET manufactured when the total concentration of impuritiesincluded in a base material is a conventional value;

FIGS. 2A, 2B and 2C are sectional views showing an embodiment of stepsin manufacturing a semiconductor device according to the presentinvention; and

FIG. 3 is a graph showing drain current-drain voltage characteristics ofthe semiconductor device of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of a semiconductor device according to the presentinvention will be described below with reference to FIGS. 2A to 2C andFIG. 3.

As shown in FIG. 2A, a base material containing boron and carbon ions ata total impurity concentration of 2 ×10¹⁷ atoms/cm³ is used for a GaAssubstrate 11. After a proper pretreatment is performed on the GaAssubstrate 11, an SiOx layer 12 having a thickness of, e.g., 5,000 Å isdeposited on the substrate by chemical vapor deposition. Openings 21 and22 are formed on desired portions of the substrate, through the SiOxlayer, using resist pattern 13, by photolithography. Si ions areimplanted in the openings 21 and 22 at an accelerated energy of 180 keVand a dose of 5 ×10¹³ ions/cm² to form electrode layers 141 and 142.Then, as shown in FIG. 2B, the SiOx layer 12 and the resist pattern 13are removed. Thereafter, a pretreatment is performed again on thesurface of the GaAs substrate 11, and an SiOx layer 15 having athickness of, e.g., 5,000 Å is deposited on the substrate by chemicalvapor deposition. In addition, an opening 143 is formed on the GaAssubstrate 11, through the SiOx layer 15, using resist pattern 16, byphotolithography. Si ions are implanted through the opening 143 at anaccelerated energy of 100 keV and a dose of 3 ×10⁻¹² ions/cm² to form anactive layer 17 in the GaAs substrate between the electrode layers 141and 142, as shown in FIG. 2C. After the electrode layers 141 and 142,and the active layer 17 are formed, a capless annealing process isperformed in an arsenic atmosphere at 850° for 15 minutes toelectrically activate these ion-implanted layers. Thereafter, a sourceelectrode 19 is formed on the electrode layer 141, a drain electrode 20is formed on the electrode layer 142, and a gate electrode 18 is formedon the active layer 17, by a lift-off method.

In the semiconductor device constituting a GaAs MESFET of the presentinvention manufactured as described above, a base material containingboron and carbon ions for serving as an acceptor at a total impurityconcentration of 2 ×10¹⁷ atoms/cm³ or more is used for the GaAssubstrate 11. Thus, the boron and carbon ions cancel the Si ions servingas a donor forming the active layer 17. As a result, the blur of thelower end of the active layer 17 is eliminated, and the interfacebetween the active layer 17 and the GaAs substrate 11 is rendered clear,thus suppressing a nonlinear operation, as shown in FIG. 3.

In the above embodiment, an example wherein a base material containsboron and carbon ions as impurities is exemplified, and the impurityconcentration is represented as a total concentration of those ions.However, when the impurity concentration of only boron or carbon ions is2 ×10¹⁷ atoms/cm³ or more, the same effect can be obtained as in theabove embodiment. As a result, a nonlinear operation of the GaAs MESFETcan be suppressed and a change in drain current does not occur overtime.

What is claimed is:
 1. A semiconductor device comprising:a GaAssubstrate prepared from a base material containing exclusively carbonions as an impurity, having an impurity concentration of more than 2×10¹⁷ atoms/cm³ ; electrode layers on predetermined portions of saidGaAs substrate; an active layer formed adjacent to said electrode layerson said GaAs substrate by ion implantation; and source and drainelectrodes respectively on said electrode layers, and a gate electrodeon said active layer.
 2. A semiconductor device according to claim 1,wherein said GaAs substrate has an impurity concentration of less than10¹⁸ atoms/cm³.